A PEM fuel cell module include a fuel cell stack. The fuel cell stack contains numerous polymer electrolyte membranes that are intended to conduct protons between the electrodes. These membranes are most conductive when the water content of the membranes is within a certain range. The water content of the membranes may increase above this range when too much water, created in the fuel cell or brought in with ambient air or a humidifier, is kept in the stack. The water content of the membranes may fall below the range when too much of the water from these sources is removed. When a cell stack is operated at a generally constant current density, the voltage of the cell stack drops when the water content is outside of the range.